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⌂ ∞ ▽ Elite on the 6502

Start and end: DrawBigLogo

[NES version, Bank 4]

       Name: DrawBigLogo                                             [Show more]
       Type: Subroutine
   Category: Start and end
    Summary: Set the pattern and nametable buffer entries for the big Elite
             logo on the Start screen

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawBigLogo_b4 calls DrawBigLogo




.DrawBigLogo

 LDA #HI(bigLogoImage)  ; Set V(1 0) = bigLogoImage
 STA V+1                ;
 LDA #LO(bigLogoImage)  ; So we can unpack the image data for the big Elite logo
 STA V                  ; into the pattern buffers

 LDA firstFreePattern   ; Set K+2 to the next free pattern number, to send to
 TAY                    ; the DrawImageNames routine below as the pattern number
 STY K+2                ; of the start of the big logo data

 ASL A                  ; Set SC(1 0) = pattBuffer0 + firstFreePattern * 8
 STA SC                 ;
 LDA #LO(pattBuffer0)   ; So this points to the pattern in pattern buffer 0 that
 ROL A                  ; corresponds to the next free pattern in
 ASL SC                 ; firstFreePattern
 ROL A
 ASL SC
 ROL A
 ADC #HI(pattBuffer0)
 STA SC+1

 ADC #8                 ; Set SC2(1 0) = SC(1 0) + (8 0)
 STA SC2+1              ;
 LDA SC                 ; Pattern buffer 0 consists of 8 pages of memory and is
 STA SC2                ; followed by pattern buffer 1, so this sets SC2(1 0) to
                        ; the pattern in pattern buffer 1 that corresponds to
                        ; the next free pattern in firstFreePattern

 JSR UnpackToRAM        ; Unpack the data at V(1 0) into SC(1 0), updating
                        ; V(1 0) as we go
                        ;
                        ; SC(1 0) is pattBuffer0 + firstFreePattern * 8, so this
                        ; unpacks the big logo pattern data into pattern buffer
                        ; 0, starting from pattern firstFreePattern

 LDA SC2                ; Set SC(1 0) = SC2(1 0)
 STA SC                 ;             = pattBuffer1 + picturePattern * 8
 LDA SC2+1
 STA SC+1

 JSR UnpackToRAM        ; Unpack the data at V(1 0) into SC(1 0), updating
                        ; V(1 0) as we go
                        ;
                        ; SC(1 0) is pattBuffer0 + firstFreePattern * 8, so this
                        ; unpacks the big logo pattern data into pattern buffer
                        ; 0, starting from pattern firstFreePattern

 LDA #HI(bigLogoNames)  ; Set V(1 0) = bigLogoNames, so the call to
 STA V+1                ; DrawImageNames draws the big Elite logo
 LDA #LO(bigLogoNames)
 STA V

 LDA #24                ; Set K = 24 so the call to DrawImageNames draws 26
 STA K                  ; tiles in each row

 LDA #20                ; Set K+1 = 20 so the call to DrawImageNames draws 20
 STA K+1                ; rows of tiles

 LDA #1                 ; Set XC and YC so the call to DrawImageNames draws the
 STA YC                 ; big logo at text column 5 on row 1
 LDA #5
 STA XC

 JSR DrawImageNames     ; Draw the big Elite logo at text column 5 on row 1

 LDA firstFreePattern   ; The big logo takes up 208 patterns, so add 208 to the
 CLC                    ; next free pattern number in firstFreePattern, as we
 ADC #208               ; just used up that many patterns
 STA firstFreePattern

 RTS                    ; Return from the subroutine